This invention pertains to free radical-curable fluoroelastomer compositions which contain coagents based on aromatic imides having pendant allyl groups for the formation of crosslinks between fluoroelastomer polymer chains.
Fluoroelastomers are items of commerce, being employed in a variety of end use applications where chemical or thermal resistance is important. They are especially useful as seals. These elastomers are normally crosslinked when formed into their final part shapes. It is desirable for the crosslinks to have at least as much chemical and thermal stability as the fluoroelastomers themselves.
One method of forming crosslinks is via a free radical mechanism, commonly initiated by such means as a peroxide or ultraviolet (UV) light. Fluoroelastomers that can be crosslinked by a free radical mechanism often contain cure sites along the polymer chain, at chain ends, or both. Typical cure sites are bromine, chlorine, Iodine or nitrile groups. See for example U.S. Pat. Nos. 4,214,060; 4,243,770; and 4,983,680. A multifunctional coagent, having multiple sites of unsaturation (e.g. triallyl isocyanurate (TAIC)), is employed to form the actual crosslinks between fluoroelastomer polymer chains.
Although TAIC is the most frequently employed commercial coagent for free radical curing of fluoroelastomers, it has some disadvantages. First, under typical curing conditions, it undergoes a competitive reaction wherein TAIC homopolymerizes, rather than forming crosslinks with the elastomer. Thus, the cure state and physical properties (i.e. tensile strength and compression set resistance) of the resulting cured fluoroelastomer part are not as good as they would have been had all the TAIC present resulted in crosslinks. Another problem with TAIC is its limited compatibility with fluoroelastomers. During press cure, TAIC may come to the surface of the shaped fluoroelastomer part. This results in fewer crosslinks within the part. Also, the TAIC at the surface may homopolymerize to form a sticky substance which adversely affects mold release. An additional disadvantage of TAIC is that it may not be as thermally stable as the fluoroelastomer that it is crosslinking. Thus, the vulcanized elastomer may be subject to thermal decomposition at the crosslinks. Lastly, TAIC is difficult to mix with fluoroelastomer compositions.
Others (U.S. Pat. No. 6,191,233 B1) have tried to overcome some of these deficiencies by partially fluorinating the TAIC, thus making it more compatible with the fluoroelastomer. However, the fluorinated TAIC is still capable of homopolymerizing and, thus, a further improvement in coagent would be welcomed.
Japanese Kokai Patent Application 63-291933 discloses the use of certain N,N-diallylbiphenyl tetracarboxydiimides as coagents for peroxide curable elastomers such as ethylene/propylene copolymers, polybutadienes, polyisoprene, butyl rubber and styrene copolymers. These coagents are more thermally stable than TAIC.
An object of the present invention is to provide a thermally stable coagent for the efficient free radical curing of fluoroelastomers wherein said coagent does not readily homopolymerize under curing conditions and wherein said coagent is compatible and mixes well with the fluoroelastomer. Accordingly, an aspect of the present invention is a free radical-curable composition comprising:
A) fluoroelastomer having cure sites selected from the group consisting of bromine atoms, chlorine atoms, iodine atoms, nitrile groups, non-conjugated dienes, and mixtures of two or more thereof; and
B) a coagent having the general formula 
where X is C(CF3)2, C(Rf)2, (CF2)n, O, CO, S, or Rfxe2x80x2; and wherein Rf is a C1-C4 perfluoroalkyl group, Rfxe2x80x2 is a perfluorooxyalkylene group having 2 to 4 carbon atoms, and n is an integer from 1 to 8.
Another aspect of the present invention is a coagent of the formula 
where X is C(CF3)2.